1. Field of the Invention
The present invention relates to a valve control system for electromagnetically actuating intake and exhaust valves of an internal combustion engine, and more particularly to a valve control system for controlling an exhaust valve when an exhaust brake is applied.
2. Description of the Related Art
Diesel engines are generally associated with an exhaust brake as a means for increasing the power of an engine brake. The exhaust brake includes an exhaust brake valve disposed in an exhaust pipe extending from the engine. When the motor vehicle with the exhaust brake runs downhill on a long sloping road, the exhaust brake valve is closed to close the exhaust pipe for thereby increasing the back pressure of exhaust gases on the engine, so that the friction loss in the exhaust stroke is increased for an increased engine brake power. If the back pressure on the engine exceeds a certain level at the time the exhaust brake valve is closed, however, the exhaust valve of the engine cylinder which is in the intake stroke is forcibly opened under the back pressure against the bias of the valve spring of the exhaust valve, thus allowing engine exhaust gases to flow back into the engine cylinder. Therefore, the back pressure on the engine should not be increased beyond the predetermined pressure level. The engine brake power is thus governed by the force of the exhaust valve spring. In order to further increase the engine brake power produced by the exhaust brake, the spring bias of the exhaust valve spring may be increased. If the spring force of the exhaust valve spring is increased, however, undue stresses will be imposed on the mechanism for operating the valves of the engine, and the cams and rocker arms will be worn prematurely.
Electromagnetic valve control systems for controlling the intake and exhaust valves of engines through an electromagnetic actuator means, rather than mechanical drive means, are disclosed in Japanese Laid-Open Patent Publication Nos. 58(1983)-183805 and 61(1986)-76713. These publications only show the operation of the intake and exhaust valves with the electromagnetic drive means.